Sintering furnace with hydrogen carbon dioxide atmosphere

ABSTRACT

A heated furnace for sintering structures of uranium oxide containing composition being introduced to the furnace. The furnace receives an atmosphere comprising a mixture of hydrogen and carbon dioxide as initially introduced to the furnace, and this mixture reacts in the furnace to give the presence of water vapor and carbon monoxide.

REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 299,222, filed Oct. 20,1972, now abandoned, which application is a division of Ser. No. 62,353,filed Aug. 10, 1970, now abandoned in favor of continuation-in-partapplication Ser. No. 358,737, now U.S. Pat. No. 3,872,022.

BACKGROUND OF THE INVENTION

This invention relates to the production of sintered uranium oxidecontaining compositions. One of the very important utilities of uraniumoxide, especially uranium dioxide, is in nuclear power plants as a fuelin the generation of electric power. The uranium dioxide, either aloneor in a mixture with other ceramics such as gadolinium oxide orplutonium oxide, is compacted to a given size and shape and sintered toachieve dense bodies for use in a nuclear fuel rod. The uranium presentin uranium dioxide must be enriched with the U-235 isotope which is donein a gaseous state, the preferred practice being to use uraniumhexafluoride. After enrichment it is necessary to convert the uraniumhexafluoride to uranium dioxide. The resulting uranium dioxide cancontain undesired fluoride ion concentrations and an oxygen to metalratio above the desired ratio of about 1.98:1 to about 2.04:1.

The sintering of uranium dioxide structures has been used to attempt toreduce the oxygen and the fluoride content of the uranium dioxide. Thecurrent practice has been the use of wet hydrogen atmospheres attemperatures preferably greater than 1600°C to achieve dense bodies ofuranium dioxide. Past experience indicates a certain amount of watervapor with the hydrogen is required to remove the fluoride content fromcompacted ceramic structures during sintering, but the wet hydrogenprocess has not been satisfactory when the ceramic has high fluorideconcentrations.

Another method presented in U.S. Pat. No. 3,375,306 for sintering denseuranium dioxide structures with or without ceramic additives is to heatthe compressed powder at a temperature of 1300°to 1600°C in a sinteringatmosphere of carbon dioxide or a mixture of carbon dioxide and carbonmonoxide and cooling the sintered structure in a reducing atmospherewhich varies as the composition of the structure varies. Where thestructure being sintered is uranium dioxide the cooling gas is dryhydrogen, wet hydrogen or a mixture of carbon dioxide and carbonmonoxide. Where the structure is uranium dioxide with an additive ofplutonium dioxide, the cooling gas is steam or carbon dioxide mixed withcarbon monoxide. The use of a mixture of carbon dioxide and carbonmonoxide is more costly than use of wet hydrogen but enables the use oflower temperatures to achieve sintered structures of high density.However this carbon monoxide-carbon dioxide sintering atmosphere doesnot appreciably decrease the fluoride content of the uranium dioxidestructures.

Sintering temperatures of about 1600°C or higher produce uranium dioxidestructures having large grain sizes with undesirable properties for somefuel applications. Uranium dioxide structures of smaller grain size havehigher creep rates when compared to the creep rates of uranium dioxidestructures of larger grain size. A higher creep rate for a uraniumdioxide structure is desirable for better fuel performance. It has alsobeen determined that other mechanical properties of finer grain sizeuranium dioxide are superior to the properties of coarser grain sizeuranium dioxide. The foregoing has made it desirable to lower thesintering temperature of uranium dioxides, and more generally ofstructures rich in uranium oxide, in addition to controlling the oxygento metal ratio of the sintered structure and removing undesirableimpurities from the sintered structure such as fluoride ions.

Lower sintering temperatures have other desirable features includingcost savings as less power is expended in heating the sintering furnace,a longer functional life for the sintering furnace and its associatedfixtures, less corrosion of the furnace components and the possibilityof adapting a continuous conveyor belt or other means of transportingthe structures rich in uranium oxide through the furnace.

SUMMARY OF THE INVENTION

Accordingly it is an object of this invention to lower the temperatureat which the sintering of structures of compacted powders rich inuranium oxide is accomplished and still obtain dense structures of thesintered ceramic.

A further object of this invention is the use of an atmosphere of amixture of hydrogen and carbon dioxide to remove impurities such asfluoride ions from compacted structures of uranium oxide containingcompositions during sintering.

A still further object of this invention is the use of an atmosphere ofa mixture of hydrogen and carbon dioxide to control the ratio of oxygento metal atoms during sintering compacted structures of uranium oxidecontaining compositions.

Still another object of this invention is the sintering of uranium oxideunder an atmosphere of a mixture of hydrogen and carbon dioxide whichreacts to give carbon monoxide and water vapor and removes fluoride ionsand other undesirable constituents from the uranium oxide duringsintering.

A still further object of this invention is the use of an atmosphere ofa mixture of hydrogen and carbon dioxide for sintering structures ofcompacted uranium dioxide powders.

Other objects and advantages of this invention will become apparent fromthe following specification and the appended claims.

The above objects, and others, are accomplished according to thisinvention by providing a striking improvement in the sintering ofceramic structures of compacted powder rich in uranium oxide involvingheating the structures at a temperature in the range of about 900° toabout 1500°C in a sintering atmosphere of a mixture of hydrogen andcarbon dioxide. The carbon dioxide and hydrogen react to give carbonmonoxide and water vapor which enables the removal of undesirableimpurities from the ceramic structures such as fluoride ions and thecontrol of the oxygen content of the ceramic. After undergoing the abovereaction, the carbon dioxide-hydrogen atmosphere controls the partialpressure of oxygen during sintering and provides water vapor promotingthe removal of fluoride ions from the ceramic being sintered.

While this process is especially useful with ceramic compositions ofuranium dioxide and mixtures of uranium oxides having an oxygen touranium ratio of up to 2.25, it is particularly useful with uraniumdioxide containing ceramic compositions with one or more ceramicadditives such as gadolinium oxide and plutonium oxide.

BRIEF DESCRIPTION OF THE DRAWING

The practice of the disclosed process will be further understood byreference to the accompanying drawings in which

FIG. 1 presents a graph of the partial pressure of oxygen versus thehydrogen content of the carbon dioxide-hydrogen atmosphere at varioustemperatures and

FIG. 2 presents a furnace adapted to receive a sintering atmosphere asdisclosed in this invention for ceramic shapes being sintered in thefurnace.

DETAILED DESCRIPTION OF THE INVENTION

It has been surprisingly discovered that compacted powders of uraniumoxide containing compositions, with or without ceramic additives, may besintered to very high density by heating to a temperature in the rangeof about 900° to about 1500°C in an atmosphere consisting of a mixtureof hydrogen and carbon dioxide. The sintering process of this inventionproduces a ceramic composition having a controlled ratio of oxygen tometal atoms, a fluoride ion content of about 25 parts per million orless and a density up to about 98 percent of theoretical density andpreferably a density in the range of 92 to 96 percent of theoreticaldensity. The sintering process of this invention, in addition tocontrolling the fluoride content, density and oxygen to metal ratio ofthe resulting sintered ceramic, removes other undesired impurities andcontamination from the uranium oxide containing compositions such asentrapped hydrocarbons, entrapped gases, greases, oils, voids, etc.

The phrase "uranium oxide containing compositions" is used herein tocover compositions capable of being sintered in the practice of thisinvention which include uranium dioxide (UO₂) and mixtures of uraniumoxide having an oxygen to metal ratio of up to 2.25 which could includemixtures of uranium dioxide with one or more of the following: uraniumtrioxide (UO₃), uranium tritaoctoxide (U₃ O₈), uranium sesquioxide (U₂O₃), uranium pentoxide (U₂ O₅), or uranium tetroxide (UO₄). Theinvention is also applicable for the foregoing uranium oxide containingcompositions with one or more ceramic additives including the plutoniumoxides such as plutonium dioxide (PuO₂), gadolinium oxide (Gd₂ O₃),titanium dioxide (TiO₂), silicon dioxide (SiO₂), aluminum oxide (Al₂ O₃)and combinations thereof. The plutonium dioxide can be present inamounts up to about 30 percent by weight of the composition; thegadolinium oxide can be present in amounts up to about 15 percent byweight of the composition; the titanium dioxide can be present inamounts up to about 5 percent by weight of the composition; the silicondioxide can be present in amounts up to about 5 percent by weight of thecomposition; and the aluminum dioxide can be present in amounts up toabout 5 percent by weight of the composition.

The composition of the sintering atmosphere can vary greatly dependingon the fluoride ion content of the ceramic and the oxygen to metal ratioof the ceramic before sintering and the ratio desired after sintering aswell as the density to be achieved for the sintered ceramic. A sinteringatmosphere in the range of about 0.5 to about 90 percent hydrogen byvolume with the balance carbon dioxide can be employed, and a preferredrange is about 2 to about 20 percent hydrogen by volume with the balancecarbon dioxide. Any source of hydrogen can be employed such as cylindersand tanks of hydrogen or a gas such as ammonia which dissociates toprovide a source of hydrogen. The sintering atmosphere can containcarrier gases such as nitrogen and argon in addition to the essentialconstituents of hydrogen and carbon dioxide.

The mixture of carbon dioxide and hydrogen constituting the sinteringatmosphere undergoes a thermodynamic equilibrium of the followingreaction: CO₂ + H₂ → CO + H₂ O. The degree of reaction between these twogases is determined by the equilibrium constant of the above reactionwhich is a function of the temperature of the atmosphere. The atomicratio of oxygen to metal of the material being sintered equilibrateswith the partial pressure of oxygen in the system with a fixed value ata given temperature. The desired oxygen to metal ratio of the uraniumoxide compositions are obtained by adjusting the partial pressure ofoxygen in the sintering atmosphere. The partial pressure of oxygen inthe sintering atmosphere can be reduced by an increase in the hydrogenconstituent of the sintering atmosphere and an increase in the partialpressure of oxygen in the sintering atmosphere is achieved by anincrease in the carbon dioxide constituent of the sintering atmosphere.

In the practice of this invention, an enhanced sintering rate resultsduring the sintering of the uranium oxide containing compositions due tothe lower activation energy required for the diffusion of the slowermoving species of metallic ions in the sintered composition. By choosinga gas composition having a mixture of carbon dioxide and hydrogendetermined by the partial pressure of oxygen desired for the sinteringatmosphere at a given sintering temperature, the uranium oxidecontaining composition can be sintered at the desired oxygen to metalratio giving enhanced sintering at lower temperatures than practiced inthe prior art. FIG. 1 presents a graph of the relationship between thepartial pressure of oxygen and the hydrogen content in the carbondioxide-hydrogen atmosphere for given temperatures. The water vapor soproduced from the reaction given above acts as a hydrolysis agent forremoving fluoride ions and when the hydrogen component is within therange set forth above the final fluoride content of the uranium oxidecomposition will be reduced to less than 25 parts per million. Thisrepresents a significant reduction in the fluoride content of theuranium oxide as commercially available uranium oxide powders suitablefor sintering can contain up to 0.5% by weight or more of fluoride ions.As stated previously, a certain amount of water vapor is needed topromote the removal of fluoride from compacted uranium oxide structuresduring sintering. The amount of water vapor in the carbondioxide-hydrogen atmosphere decreases as the gas composition is selectedfrom a carbon dioxide rich or a hydrogen rich mixture. But relativelysmall amounts of water vapor in the sintering atmosphere disclosed inthis invention achieves a much greater reduction in the fluoride contentat the same sintering temperature than can be achieved using a sinteringatmosphere without the presence of water vapor (e.g., dry carbon dioxideor dry hydrogen).

The following is representative of the preliminary processing of theuranium oxide powder into pellet form before it is introduced to thefurnace, but this is in no way meant to be a limitation on the teachingof this invention. The uranium oxide employed in the process of thisinvention can be commercial grades of the ceramic having up to 0.5% byweight fluoride ions or greater and varying oxygen to metal ratios. Theuranium oxide in powder form is pressed in a press at pressures up toabout 10,000 pounds per square inch into desired shapes, such as smallcylinders, cubes, parallelepipeds, etc. These desired shapes are thengranulated in a granulator and screened through a screen having openingsin the range of about 5 to about 20 mesh. This processing sequenceincreases the flow properties and bulk density of the uranium oxidepowder. The granulated powder is then pressed into desired shapes suchas small cylinders, cubes, parallelepipeds, etc., in a press withapplied pressure up to 40,000 pounds per square inch. Representativedimensions of cylindrical shapes of pellets made by this invention arepellets having a diameter of about 1/2 inch and a height of about 1/2inch.

The invention can be applied in a batch process where the pellets ofuranium oxide are placed in a suitable container in a cold furnace,heated to temperature under the hydrogen-carbon dioxide atmosphere asdescribed in this invention and maintained at this temperature for aperiod of from about 1 to about 5 hours followed by cooling the pelletsin the same atmosphere or another atmosphere such as a different mixtureof hydrogen and carbon dioxide, wet hydrogen or dry hydrogen. Anotherembodiment of the process of this invention as shown in FIG. 2 hasplacing of the uranium oxide pellets 12 in a series of ceramic boats 11placed in a furnace generally represented by number 10 having a gasoutlet 16 going to an exhaust. The furnace has three zones oftemperature as generally indicated in the drawing and designated apreheating zone, a sintering zone and a cooling zone. The boats 11 arepushed through the furnace 10 by introduction of additional boats 11.Each boat 11 passes through door 21 to the purge chamber 20 with door 21then being closed followed by lifting of door 13 so the boat can bepushed into furnace cavity 22 by a device such as a pushing rod (notshown). The first region through which the boat passes is a preheatingzone which has an increasing temperature as the boat moves closer toheating elements 14. In the preheating zone the atmosphere of thefurnace 10 is a mixture of hydrogen and carbon dioxide as taught in thepractice of this invention. It is thought that an appreciable amount ofthe fluoride and oxygen to be removed from the pellets 12 isaccomplished before the ceramic boat reaches the sintering zone. Whenthe boat 11 reaches the sintering zone, the temperature is maintained inthe range of 900° to 1500°C during which a substantial proportion of thesintering practiced in this invention occurs. The atmosphere in thesintering zone is the same as maintained in the preheat zone with theatmosphere varying between about 0.5 to about 90 percent by volumehydrogen with the balance being carbon dioxide. When a boat reaches thecooling zone the temperature falls as the distance from heating elements14 is increased. In FIG. 2 there are shown two gas inlets 18 and 19,with gas inlet 18 being located toward the heating element 14 while gasinlet 19 is located toward the door 17 which could be a lift dooropening into purge chamber 23. Door 24 is opened to enable removal ofboats 11 from purge chamber 23. As previously stated the atmosphere inthe cooling zone can be either dry hydrogen, wet hydrogen or a mixtureof hydrogen and carbon dioxide within the range disclosed in thisinvention. Where dry hydrogen or wet hydrogen is maintained in thecooling zone, the hydrogen gas is fed into inlet 19 from a tank 25 withcarbon dioxide being fed into the furnace in gas inlet 18 from tank 26,both gases being fed into the furnace in amounts sufficient toultimately form a mixture as disclosed in this invention. Where amixture of hydrogen and carbon dioxide is maintained in the coolingzone, both hydrogen and carbon dioxide are fed into inlet 19 from tanks25 (hydrogen) and 27 (carbon dioxide) with inlet 18 being closed. If adifferent mixture of carbon dioxide and hydrogen is desired in thecooling zone than in the sintering zone, the cooling zone mixture is fedinto inlet 19 from tank 25 (hydrogen) and tank 27 (carbon dioxide) andthe sintering zone mixture is fed into inlet 18 from tank 26 (carbondioxide) and tank 28 (hydrogen). From the figure it can be seen that thesintering atmosphere flows in the direction opposite the direction theceramic boats are being moved through the furnace. This insuresmaintaining the desired oxygen to metal ratio and the removal offluoride content from the sintered uranium oxide structure. Thesintering atmosphere can flow in the same direction in which the ceramicboat is being moved if desired but a higher flow rate of gas ispreferred for this arrangement. The flow rate of gas through anexperimental tube furnace would be in the range of about 1 to about 10cubic feet per hour in a furnace having a heating chamber 22 of 1 1/2inches in diameter and 3 feet in length.

The heating means of the furnace can be commerically available heatingelements such as resistance heating elements and induction coils. Thefurnace walls can be made of ceramic such as alumina or high temperaturemetals such as high temperature stainless steels.

A stoichiometric uranium oxide product, for example, a uranium dioxideproduct in which the ratio of oxygen atoms to metal atoms issubstantially within the range of about 1.98:1 to about 2.04:1 andpreferably about 2.00:1, may be produced by cooling the sinteredmaterial in an atmosphere such as dry hydrogen, wet hydrogen or amixture of carbon dioxide and hydrogen. It is possible to select amixture of hydrogen and carbon dioxide such that preheating, sinteringand cooling can be carried out in the same atmosphere to achieve highdensity and an oxygen to metal ratio of substantially about 1.98:1 toabout 2.04:1.

The discovery of this invention has the advantage of sintering uraniumoxide structures at lower temperatures of about 900° to about 1500°C,thus avoiding the necessity for heating at temperatures in excess of1600°C when utilizing a wet hydrogen atmosphere for sintering uraniumoxides. The present invention also has the advantage of controlling thepartial pressure of oxygen in the furnace by controlling the relativeproportions of carbon dioxide and hydrogen maintained in the furnace.

Another feature of this invention is the range of processing parametersenabling a person skilled in the art great flexibility in selecting theprecise operating parameters depending on the properties desired for thesintered uranium oxide containing composition.

The sintering atmosphere disclosed in this invention is eithercomparable or cheaper in cost than gas atmospheres currently beingutilized in commercial sintering of uranium oxide structures. Thesintering atmosphere of this invention enables the use of lower furnacetemperatures with lower operating costs for heating the furnace andgives a longer furnace life due to less corrosive conditions associatedwith fluoride containing atmospheres at lower furnace temperatures.

The teaching of this invention and the methods by which it is to beperformed will become apparent from the following examples which areoffered to be illustrative of the invention but are not to serve as alimitation of the teaching of this invention.

EXAMPLE 1

A batch of cylindrical structures of compacted uranium dioxide powder ofabout 0.5 inches in diameter and 0.6 inches in height is placed in analumina boat and charged to a cold furnace. The furnace is heated to1100°C in 2 hours and maintained at that temperature for 4 hours in anatmosphere of 10% by volume hydrogen with the balance being carbondioxide. The same atmosphere is maintained throughout this heatingcycle. The structures are then cooled in dry hydrogen to roomtemperature giving a structure having an average density of 95.0 percentof theoretical density and an oxygen to uranium ratio of 2.006. Theinitial fluoride ion content of the structures before sintering is 570parts per million and the fluoride ion content of the sinteredstructures is 5 parts per million.

EXAMPLE 2

Another batch of cylindrical structures of compacted uranium dioxidepowder (about 0.5 inches in diameter and 0.6 inches in height) is placedin an alumina boat and charged to a cold furnace. The furnace is heatedto 1100°C in 2 hours and maintained at that temperature for 4 hours inan atmosphere of 5% by volume hydrogen and 95% by volume carbon dioxide.The sintered pellets are then cooled to room temperature in the sameatmosphere giving a structure having an average density of 97.2 percentof theoretical density, a final oxygen to uranium ratio of 2.007,fluoride ion content of six parts per million, entrapped gas in theceramic of 3.7 microliters per gram and a grain size of about 1 micron.Before sintering, the fluoride ion content of the cylindrical structureswas 134 parts per million and the oxygen to metal ratio was 2.085.

EXAMPLE 3

Another batch of cylindrical structures of compacted uranium dioxidepowder (about 0.5 inches in diameter and 0.6 inches in height) having afluoride ion content of 151 parts per million are placed in an aluminaboat in a cold furnace. The furnace is heated to 1300°C in 2 1/2 hoursand maintained at that temperature for 4 hours in an atmosphere of 10%hydrogen by volume with the balance being carbon dioxide, thisatmosphere being maintained throughout the heating cycle. The structuresare then cooled in the same atmosphere to room temperature giving astructure having an average density of 95.7 percent of theoreticaldensity, an oxygen to uranium ratio of 2.012 and a fluoride ion contentof 2 parts per million.

EXAMPLE 4

A batch of uranium dioxide powder (fluoride ion content of about 80parts per million) was mixed with an additive of three percent by weightgadolinium oxide and the powder was pressed into cylindrical pellets ofabout 0.5 inches in diameter and 0.6 inches in height. These pelletswere sintered at 1425°C for 4 hours at this temperature in an atmosphereof 20% hydrogen by volume with the balance of the atmosphere beingcarbon dioxide. This atmosphere was maintained throughout the heatingcycle and during cooling to room temperature. The sintered structure hasan average density of 95% of theoretical density and a fluoride contentof 5 parts per million.

It is to be understood that, although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited, since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

What is claimed is:
 1. An enclosed heated sintering furnace forcontrolling the partial pressure of oxygen in the furnace and forremoving impurities from structures of uranium oxide containingcompositions being sintered in the furnace, the furnace having means forintroducing an atmosphere comprising a mixture of hydrogen and carbondioxide as initially introduced to the furnace and the furnace havingmeans for passing the structures countercurrently to the atmosphere. 2.An enclosed heated sintering furnace according to claim 1 in which thereis from about 0.5 to about 90 percent hydrogen by volume and the balanceis carbon dioxide.
 3. An enclosed heated sintering furnace according toclaim 1 in which there is from about 2 to about 20 percent hydrogen byvolume and the balance is carbon dioxide.
 4. An enclosed heatedsintering furnace according to claim 1 in which the atmosphere has inaddition a carrier gas.
 5. An enclosed heated sintering furnaceaccording to claim 1 in which the carrier gas is nitrogen.
 6. Anenclosed heated sintering furnace according to claim 1 in which thecarrier gas is argon.
 7. An enclosed heated sintering furnace accordingto claim 1 in which the hydrogen in the atmosphere is from dissociatedammonia and the atmosphere has a carrier gas of nitrogen.
 8. An enclosedheated sintering furnace according to claim 1 in which the interior ofthe furnace is maintained at a temperature in the range of about 900° toabout 1,500°C.
 9. An enclosed heated sintering furnace according toclaim 1 in which the furnace has therein structures comprised of uraniumoxide with fluoride impurities.
 10. An enclosed heated sintering furnaceaccording to claim 1 in which the furnace has therein structurescomprised of uranium dioxide with fluoride impurities.